A so-called one-plane 180° type engine, that is, an engine with crankshaft arrangement in which crank pins of first and fourth cylinders are arranged in a same phase while crank pins of second and third cylinders are arranged with a 180° phase difference has been widely used as an engine for a vehicle conventionally.
JP-A-57-69137 and JP-A-9-250597 propose an engine for a vehicle in which a balancer (referred to as a primary balancer) for constant velocity is provided for a crankshaft whose arrangement of each crank pin is, respectively, 0° for a first cylinder, 90° for a second cylinder, 270° for a third cylinder and 180° for a fourth cylinder (referred to as a two-plane 90° type).
In a vehicle such as a motorbike provided with an engine using the above-mentioned two-plane 90° type crankshaft, a driver can feel driving torque from the engine more strongly than in the case of an engine using the former-mentioned one-plane 180° type crankshaft, from a point of view of S/N (SN ratio) of the driving torque, amplitude of vibrations, frequency and such. Thus, acceleration feeling, and thereby, a driving sense are improved. Therefore, it is strongly desired to put the two-plane 90° type engine for a vehicle into practice.
An engine in JP-A-57-69137 is provided on the premise that the total weight added to a crank web is set by adding ½ of the weight Wrec of a reciprocating portion to the weight Wrot of a rotating portion (refer to a formula (12) in the left column of Page 3). That is to say, Wrec/2 (additional weight, unbalance weight) is added other than the weight Wrot balanced with the weight Wrot of the rotating portion such as a large end portion of a connecting rod. Further, it can be seen that calculation is performed on the assumption that the weight of a crank web of each of the cylinders concentrates at the center of the cylinder, since the distance from the center of a crankshaft to the center of each of the first and fourth cylinders is set at 2a, while the distance to the center of each of the second and third cylinders is set at 2b (refer to FIG. 1).
The crank web of each of the cylinders is actually divided between the left and the right with respect to the center of the cylinder. Accordingly, in order to assume that the sum of the halves of the crank web concentrates at the center of the cylinder, the additional weight of each of the half crank webs should be ¼ of the weight of the reciprocating portion. The engine in JP-A-57-69137 is provided on the premise that the additional weight of a whole crank web for each cylinder is ½ of the weight Wrec of the reciprocating portion, namely, the additional weight of a half crank web is ¼ of the weight of the reciprocating portion.
In an actual engine design, however, it is often difficult to set the additional weight of a half crank web at ¼ of Wrec since interference with other gears, bearings and the like in the vicinity of the crankshaft should be prevented. To achieve the same effect, enlargement of the rotational radius can also be considered. In this case, however, the crankshaft becomes large.
In the engine proposed in JP-A-9-250597, it is assumed, similar to the engine proposed in JP-A-57-69137, that the additional weight added to a crank web is ½ of the weight Wrec of the reciprocating portion (in Paragraph 0022), and that the weight of a crank web for each cylinder concentrates at the center of the cylinder (in Paragraph 0018). Accordingly, the additional weight of each half crank web is assumed to be ¼ of the weight of the reciprocating portion as a premise. On the basis of this premise, in JP-A-9-250597, an isotropic balancer rotating at a constant velocity in an isotropic direction with respect to the crankshaft is provided, in addition to a reverse balancer rotating at a constant velocity in a reverse direction with respect to the crankshaft, and a part of the weight Wrec/2 added to the crankshaft is divided for the isotropic balancer so that the addition weight of the crankshaft is made ½ of Wrec or less. In this case, however, a balancer rotating in the isotropic direction and a balancer rotating in the reverse direction cause unavoidable increase in size of the engine.